Manufacture of large glass structures, in particular astronomical telescope mirrors



jam. 9, 1951 Filed July 16, 1949 J. JOBLlNG-PURSER ET AL ASTRONOMICAL TELESCOPE MIRRORS E. MANUFACTURE OF LARGE GLASS STRUCTURES, IN PARTICULAR 2 Sheets-Sheet l 1951 E. .LJOBLING-PURSER ETAL 2,537,465

MANUFACTURE OF LARGE GLASS STRUCTURES, IN PARTICULAR ASTRONOMICAL TELESCOPE MIRRORS Filed July 16, 1949 i I K 6 V :r G A 3 5 F l I m hwy r F fink Rug s 5 R32 4 4 xx 7 -33 5 AVAVA J 522 V AV AV 6 E2 5% V L lug l g" 1 FIGJI'.

a. *5 x XE G Patented Jan. 9, 1951 v MANUFACTURE'OF LARGE GLASS STRUC- TURES, IN PARTICULAR ASTRONOMICAL TELESCOPE MIRRORS Ernest Joseph .lobling-Purser, Burdon Hall, near Sunderland, and Shaun Maturin Cox, Westoe Village, England, assignors to James A. J obling & Company Limited, Sunderland, England, a company of Great Britain Application July 16, 1949, Serial No. 105,142 In Great Britain April 22, 1948 14 Claims.

1 The invention relates to the manufacture of astronomical telescope mirrors and other large and integrate glass structures.

The problems which it is the aim of the invention to solve have arisen chiefly out of investiga- 5 ing of the disc, and so long as such distortion is tions made in connection with the fabrication of present the figure of the mirror is inaccurate. astronomical telescope mirrors and, for simplicity, Under observatory conditions it is impossible to the invention will be described in that connection. maintain uniform ambient temperature, for the It is to be understood, however, that this does mirror must be exposed to the skyusually at a not imply limitation of the invention to that field site such as a mountain top, where extremes of alone. day and night temperature arise. A mirror of 2" It is well known that the extent of the field of sectional thickness may take two or three hours the astronomers enquiry is set by the aperture to acquire night temperature. A mirror of of his telescope and in fact both the light collecttwice this thickness will take four times as long ing power and the detail of what can be seen or to acquire night temperatureso that there is photographed is set by the size of the mirrors or little time to make observations before dawn. lenses of the telescope. Because of the difficulty Suggestions have been made and experiments of'obtaining large optically homogeneous pieces of have been carried out'and published with regard glass, it has become the practice to use mirrors to the partial solution of this problem. Thus the exclusively as then the optical requirements are 200" disc at presentunder trial at Mount Paloreduced to the necessity of perfection in surface mar was cast with a ribbed structure at its back. contour for reflection, the material part of the Such a large intricate casting was exceedingly mirror simply serving as a support for this recostly to manufacture and was only successfully fleeting surface. 7 cast at the second attempt. Ingeneral, imperfections in this surface con- Experiments carried out in France were based tour exceeding about 2 millionths of an inch are on the formation of a composite mirror made by intolerable, and because this degree of perfection joining together two large plates of glass, sepam'ustbe maintained under all the changin condirated by glass spacer pieces. The joints were tions of support and of temperature while the. made to adhere by the use of cement of untelescope traverses the sky, the support for the specified nature, and the use of such cement reflecting surface must conform to the most exthrows doubt on the permanency and stability acting conditions of rigidity and -thermal and of such a mirror. temporal stability, A somewhat similar arrangement was resorted As far as stability is concerned considerations to by Sir Charles Parsons, who, however, caused such as the smallness of the thermal expansion the joints to sinter together by heating the W coefficient together with the exclusion of any the warping of the back plate being reduced by possibility of equilibrium changes and the need supports afterwards removed. His mirror showed for a perfect surface polish, have indicated small cracks at some of the joints-possibly the that the material best suited to the making of the result of devitrification of the glass. mirror is glass. In these composite mirrors it was envisaged To maintain adequate rigidity it is found that, that the air would permeate between the front as the diameter of the mirror is increased, the and back plates and so assist in bringing the glass supporting thickness must also be increased, and to uniform temperature. Because of the con--' for the larger mirrors the glass manufacturers tinuity of the back plate, howeve the r is l able task of casting becomes formidable. Apart from to become stagnant and itself acquire temperathe detailed difiiculties of producing and casting ture gradients. Sofar as we are aware neither large homogeneous melts of glass, the interrupof these composite mirrors reached the stage of tion of normal routine operations in the glassactual trial in a telescope. house makes such an undertaking a very expen- In all ca s the problem of produ a Satissive one to the glass manufacturer. factory large front plate still rests with the glass There is moreover an intrinsic difficulty in conm er, d u e the y a ufacturers nection with such large mirrors. The conditions equipp d t mak large di p s ave b n for rigidity and for thermal stability become plate glass makers it has been necessary to make ever more conflicting as the dimensions increase. these mirrors from plate glass which has a therl'hick sections are necessary for rigidity, but mal expansion coefilcient' of about 9 10- per 2 thick sections are detrimental to the disc becoming uniform in temperature as the ambient temperature changes. Any lack of uniformity in temperature will be apparent as a temporary warpeasiest C., or some three times greater than that of more appropriate glasses such as Pyrexf brand glass, The gain in thermal resppnse ohtainedinf design has therefore been largely off-set by the use of material of high expansion We have devoted our attention to the development of a technique which would enable us to manufacture mirror blanks which:

(a) Would have adequate rigidity.

(1)) Would have rapid temperature response Would involve the light disruption to standard glass-house practices, r

(d) Would not suffer from any deteriorate with time.

In order to comply with (chit was decided to design a composite mirror blank; ieEeS of which could be pressed in the conventional ner, in Pyrex brand glass.

The advantages of so doing are as follows:

-.(l) cThe-g1as5. iorrtheindividual pres s. b ingrgathered from a large tank of moltenwglass under .welluestablished .n i 9. 1. 9. fi f t fl would egrhibit a constancy in icomposition irom one pressing t0. anothernunaehievable were the glass. melted for, each. individua pr n we ,7

e,,(2)i-Any@p eee .containin a .fiaw could be excluded .without ,condemning a large weight of glas as .WQu1d be the case if the mirror blank were .castuasonepiecei 4 l v. v ,7 i

Inorder o. comply with .(d) itwas. decided that he piecesvshoiild be ioinedltogetherpby sintering without, thevuse ,oi arcemenhv so that the mirrorywhenwcompleted would be integrated into a single, glass structural 1 lnuordento complyiwithxaiand (b) the rear of. the .mirrormust haveadequate overall thickness, and 1 must -.have ..a.n Jopenwork structure allowing oiitheireacirculationoi, air, yet the Vari0l1S paIit-S must..,be sointerconn'ected as to ensure,..rigidity.. w'lhey parts. should at, no place have great thicknessand there should bea high proportion of,.void-vo1ume tocreducenweight.

It was by no meanseobviollsthat such -conditionscollld helmet. MByNpreliminaryexperiments it was necessary toascertain whether; a

ll A struc urenoi cqntactinenartsrcouldube made .toiuse "together .at ,the contacts by uni. rormly. heating. the whole withoutlalflthe Whole softeningto the extent that. the structure would collapse orrib y the glass devitrifyingunderthe action of the heat, and developing flaws from thecrystallinewprcductsoi devitrification, r .n (2.).NA continuous. front plate r .could be formed by,heating together smaller pieces in approximatelcontactmw. i .,,-It.,,w as tfurther appreciated, that. in view of the closeheating control which, would obviously be necessary,,it was to be desired that both these at a relatively greater rate with respect to teni perature increase than does the crystal growth velocity. As vve Wish glaisssections in ,eontact to seal together -the falc'ility for so doing is measured by the mobility-while avoiding crystalline growth, it is evident that such an operation vvill hejavoured by using as high a temperatui'e as ma {be consistent with our other reduirerheiits; It became evident, however, that the consideration that the structure should not collap b'biild oiil' be met if the temperature eereg tm ea eel-ow that point at which troublesufrorri, or htlallisation were still to be feared.

Asit is from the front plate that the final prec' contour must be formed by grinding and posh g; it i s" essential that a homogeneous arid continuous plate of the requisite diameter shall he .formed, and to overcome the manufacturing difiiculties before referred to, it is necesfiery-in thei a erdmie q to m liel li s cfrgm ma er.li ie es-vv F rt ermo e o 1 .isaihe 06st and labour of r n in it isdesw. nea he 91 91 o h s Plate should Coiiform approximately to thatfinally required.

processesr-the formingwofl the front plate and the iorming, ,oi .the .rear A openwork gstrucjturee should .be accomplished. in. an electric furnace of .the resistor .v type, and; that on (the grounds of, economy,,..the. electric elements should be .inexpensive. .In .short,v.itt was. desirable, that the fusionshould beaccomplished at temperatures less, thanullflfli C., a maximum set by the top working temperature oibasefmetal resistor Wires.

t Having now outlinedthe .difficu'lties to be overcome, we.'give below the methods by which We have achieved our object. 7 l .7 Measurements carried out on the viscosity of thegla'ss and ,uponcther rate .of vcrystaljgrowth arising from devitrification showed that while both dependupnithe, m ratu eth smo ilit y (i. e. the reciprocal of the viscosity) increases ic turissnee e err tha .thaseem i1, .Q i mew be sub ntiall te mr eieen r. 4 95 1 .bandu 'qntinuqq rews; a rth tasvnimeir e d ii r sul n item-e l m 9i imim ll pf, stru wr. .weu i sult .i i 9n On the grounds of economy of moulding eguip: m e it i ldes iteble-i nt llt ieieq se ie fi make. p i e teh9 l iha 12 em share h e, p "peer a ,re y ppalvms a e o m l t amwayiesi aee a z nd agent which are nn itually fitti their??? 9 t e .er ed p s le y, a ee twir in and b u it ead'sikrdistentur a eer na ri'n 712 b r mi' e e -M, t, ei ih r es a difi t r i a a i f u a1' a r {9 p. 5 ie n r rfih it hi ii l gee'e P e he efiufi j i si it a i 'matiq hepausedto have acohcav' face, and that the sides b'e tapered back so that the areafof the face which is concave gi'eater thanthat :of reverse sidewhichisconvex, and that if thfei aihouiit oitape'r is so en that the total volufrne'otthe gras is these aura-111a tog or' glint greater tha'n thevoliune which would ta cupied e like polygonal piece joi fth'e atire front side length r th Sam t i k ie' ni e. f es/w r th p e and the sides u'nt pred, then sucha treat, it "placed with its edges in contact with the ppropriate number otother piece of 'like shape f e Genie fgf e v l m es/are arrang'ed to allow 16: theeigpir their, theas'sei'nbly will; when he fed tda jsufficiently high temperature, soiteii aiid eolia'ps'e jt'o tlie refractory "plate and in the processor so doing the "seams between nie w ll aluni m res i el .I fb 1 upwards without the inclusion "or large air hubbles'o i ters, V .u v

We. and, that s ts a any fr i ai' rites showing only immaterial "evidence of their dis- 'e fo mfmay.i dein,fi t im k Th irregular boundary to such plates ma be rendered circular either by; subsequent cutting or by cuttingfsuch individual 'polygonais as may be n'c ssary to'conforni with the retiuird bouiid'ar'y'bc fore the fusion operation,

r The refractory mate upon which the 'pices restduring fusion lbe to tiie pp; me em rate rl s it s eviewer then}? sha l cen'i r aa d it this 59 v reat i reil the straight sides to the polygonal must be modified to have a curved shape so that when they are laid upon the refractory they lie in closer juxta-position.

Assuming the preformation of the plate, a further feature of our invention resides in increasing the rigidity of the plate by sealing thereto a system of interconnected parts to constitute a reinforcement and there is evidently an infinite variety; of designs which would achieve the purpose. H

We propose to describe one such design in detail without implying that our technique is limit ed to suchthe essential feature of our reinforcing technique lying in that the reinforcing parts are so shaped that when placed in position on the preformed plate and heated in a furnace they do not tend substantially to lose that shape, or

when they do so, the loss of shape is such as to I bring those parts which are to be fused together into more intimate contact, and further that the resultant forces due to the weight of the parts themselves or to the action of spring pressure are such as to aid the sealing together of the contacting parts under the action of heat.

To assist in a clear understanding of our invention we now make reference to the accompanying drawings, in which:

Figure l is a cross-sectional view of one of the several glass polygonal pieces used in making a mirror blank, the view being taken on the line II iri Figure 2:

Figure 2 is a plan view corresponding to Figure 1;'

Figure} is a sectional view illustrating several of the polygonal pieces arranged in juxtaposition on a refractory support prior to heating;

nigare i is a side elevational view to an enlargcd scale of one of a plurality of glass tripods used 'asipart of a reinforcement of the mirror blank" made from the aforesaid pieces, the view being 'taken' from the direction of the arrow IV in Figure 5;

Figure 5 is a plan view corresponding to Figure 4; If."

I Figure 6' is a side elevational view looking in the direction of the arrow VI in Figure 5;

Figure 'I is a vertical sectional view taken on the'line of Fig. 8 of one of a plurality of glass cups adapted to be applied to the tripods of Figures 4 to 6 as a further part of the reinforcement; Figure 8'is a plan view corresponding to Figure'l;

Figure 9 is a plan view illustrating the mirror blank with the reinforcement constituted by the tripods of Figures 4 to 6 and cups of Figures 7 and 8 applied thereto;

the concave and convex faces each-being-ciirvedto a radius of thirteen and three-quarter inches in the section which cuts the piece through 0pposite corners as indicated bythe line II in Figure 2, and a radius of ten-and-one-half inches. in the section which cuts the piece so as to bisect perpendicularly the opposite pairs of straight edges. With these dimensions, a tapering backv of the sides d at an angle of 6 from the vertical is. found satisfactory. given by way of example only and are not to be regarded as'iimitative.

As illustrated in Figure 3, a plurality of the: pieces described with reference to Figures 1 and. 2-the number depending on the ultimate size of.

the mirror blank to be made-are placed upon a plate 5 of refractory materialwith the concave" face 2 nearest the plate, the pieces being so arranged that the loweredges of theirsides l fit against one another somewhat after the manner.

I to allow air trapped in the cavities formed be- Figures 10 is a perspective view illustrating the mounting of the cups on the tripods, and

Figure 11 is a diagrammatic plan view having for its purpose to give an idea of the sizes of the is initially formed with a concave face 2 and a' convex face 3, the sides 4 of the hexagonal being tapered back so that the area of the'concave face 2 is greater than the area of the convex facet. In a specific instance, the piece can be of such a size that the apices of its sides 3 at the points where they meet the concave face 2 touch an imaginary'circle of six inches radius, the piece being two inches thick, at its vertical centre and pieces collapse onto the plate. The upper surface of the refractory plate 5 on which the pieces,

rest is, for illustrative purposes, shown to be fiat so that the mirror blank formed by the collapsed P and amalgamated pieces i will have an operative surface of corresponding fiat form for final grinding and polishing. Figure 9 will serve to demone strate how the pieces I are fitted together honeycomb fashion prior to heating as already explained, although. this figure is intended to show but which has disappeared in the collapsing of the pieces on to the refractory plate 5 and the consequent bringing of the sides into full contact with each other.

The shape at the boundary of the formed1nir-. ror blank can be governed by using at the bound.

ary pieces 5 which are of only part-hexagonal shape or are of full hexagonal shape and are trimmed downafter the joining operation.

The reinforcement for the back of the mirror blank formed as explanied with reference to Figures 1, 2, 3 and 9 will now be described. The

reinforcement consists of two layers of elements, the first of these being in the form of glass tripods adapted to rest on and be fused to the rear surface of the mirror blank and the second being in the form of glass cups adapted to rest on and.

be fused to the heads of the tripods. The tripods are generally indicated at i in the drawings and the cups are generally indicated at 8.

One of the tripods is illustrated in Figures 4 to 6. It is formed by moulding and has three symmetrically arranged splayed legs 9 each of substantially circular cross-section, the legs be-v ing united at the top in such a manner as to provide a substantially hemispherical seating l@. The legs are joined by webs ii the lower ends of which terminate above the lower ends of the legs The inner part of the lower extremity of each le isiformedas part .of a sphere as indicated at 12.

These dimensions arev while the outer part of the lower extremity is formed with two chamfers l3 disposed vertically and extending in different angular planes so that they lie at an angle of 120 to each other as indicated by the arrow line a in Figure 5. With this arrangement, when a plurality of the tripods is set up on the back surface of the mirror blank with the legs of one in contact with the legs of another, the chamfers I2 on the legs of one tripod will mate snugly with the chamfers on the legs of adjacent tripods, the contacting legs together providing a substantially hemispherical surface in contact with the mirror blank-this surface albeit being divided into three segments. Figure '9 clearly demonstrates the mating arrangement of the'tripods.

One of the cups 8 constitutingthe second layer of the reinforcement is 'illustrated in "Figures 7 and 8. The cup is made by'moulding and is of substantially triangular shape in plan with vertical flat sides 14 and a fiat base 15 into which the 'sides curve as .indicated at It in Figure 7, the cavity 11 in the cup being tapered inwardly *towards the bottom. Each corner of'the triangle is formed with two chamfers i3 disposed vertically and extending in different angular planes so that they lie at an angle of 120 to'each other as indicatedby the arrow lines bin Figure'B. Withthis arrangement, when a plurality of the cups is placed'togcther corner to corner, the 'chamfers N3 of adjoining cups will mate snugly with each other as indicatedin Figures -9 and 10. Arecess I19 'is provided at the bottom of each 'pair of chamfers 18, the recess conforming to a part. of a sphere whose radius is slightly larger than that ofthe part-spherical seating Ill at the 'top'of the tripod 1. Each cup 8 is made to lateral dimensions such that when a plurality of the cups is placed corner to corner with their cham-fers 1-8 mating, the three'recesses i 9 at each mating 'corner will together produce a substantially "hemispherical depression in which the substantially hemispherical head H! of a tripod I can fit. In thisway, when the'tripods 1 have been correctly sited in contact on the rear face of the mirror blank and'the cups 3 have been correctly arranged in contact with each other and in-engagement with the heads H) of the tripods beneath them, one set of reinforcements will act to locate the other. ihe nature of the engagement between the several parts is clearly depicted in Figure '10.

With the reinforcing layers constituted by the tripods and cups applied to the mirror blank as described and with the mirror blank resting 'on the refractory plate '5 or a similar Support, the whole assembly is heated to a degree sufficient to cause the tripods to be fused to each other to the'mi-rror blank and the cups to be fused to each other and to the tripods.

The reinforcement constituted by the tripods and cups at the back of the plate endows the latter with all necessary strength and resistance to distortion without increasing the total bulk of the plate excessively. At the same time the arrangement of tripods and cups provides a plurality of cells at the back of the plate through which air can freely pass. These factors together will ensure that the plate will quickly respond to changes of ambient temperature.

Figure 11 is a plan view depicting a mirror blank of twenty inches in diameter, the crossing lines in the figure being intended to indicate the cups .8 and the underlying tripods I of the reinforcement. This figure is included merely to give some idea of the size of the cups and tripods relatively to a mirror blank of the diameter just stated. I

With such a design of reinforcement and because of the splaying of the legs of the tripods when under the softening action of heat, the tripods tend to squat under their own weight and under the weight of the superimposed layer of cups and in so doing the adjacent surfaces are driven into contact with each other and into' contact with the mirror plate, thereby promoting the joining operation.

Because, during the fusing operation, there is little force tending to unite the contacting surfaces of the cups, spring clips 20 may with ad vantage be applied to the cups as indicated in Figure 10, each clip spanning the two walls of neighbouring cups, amounting to three clips for each cup. It is found that such clips if of suitable material such as ni'chrome wire may be removed after the fusion operation without damage to the glass.

While the design of the components of the reinforcing structure is important, a factorcontributing to the success of its application to the mirror blank is the skill exercised during the fusing operation. Because of the danger of de-' vitrification it is necessary that all parts of the structure should be brought rapidly to the fus ing temperature, and maintained there only just so long as to permit thedesired degree of squatting of the tripods. I

When the glass is Pyrex brand glass there'is little risk of either devitrification or warping at temperatures below 600 C. The parts may therefore be heated slowly untilthis temperature is reached and so any risk of breakage by ther-. mal shock can be avoided. Thereafter, because the viscosity has decreased, there is little risk of thermal breakage and therefore the temperature, should be rapidly raised to about 830 C. As at" this temperature the squatting rapidly takes place, no time can be allowed for the whole structure to become uniform in temperature, but

rather the temperature must be rapidly re duc'ed to set the structure and allay the squatting.

It is of importance therefore that the method of heating should be so devised that there are no great temperature differences throughout the structure. We find that by lining "the furnace. walls with numerous electrically heated wires so that the heat is supplied substantially as radiant energy, such natural temperature gradients may be reduced to tolerable proportions.

We claim:

'1. Method of manufacturing glass plates which resides in providing a plurality of glass pieces each of polygonal shape and with a concave face and having its sides tapered back so that the area of the concave face is greater than that-of the opposite face, placing such pieces on a .refractory support with their edges in Contact with. each other and with their concave faces nearest the support, and heating the assembly to cause,- the pieces to soften and collapse on to the .support and the sides of one piece concomitantly to come into engagement with the sides of adjacent pieces so that such sides will be joined together. from the bottom upwards.

2. A method according to claim 1, wherein .each piece has .a convex face opposite the concave face.

3. A method according to claim 1 wherein; each piece is of hexagonal shape.

4. A method according to claim .1, whereinthe refractory support is perforated to allow of escape of air occluded between the concave faces of the pieces and the adjacent face of the support when the pieces collapse.

5. A method of manufacturing a glass plate, including the step of applying a reinforcement to the back of the plate formed by the method claimed in claim 1.,

6. A method of manufacturing a glass plate which resides in providing a plurality of glass pieces each of polygonal shape and with a concave face and having its sides tapered back so that the area of the concave face is greater than that of the opposite face, placing such pieces on a refractory support with their edges in contact with each other and with their concave faces lying nearest the support, heating the assembly to cause the pieces to soften and collapse on to the support and the sides of one piece concomitantly to come into engagement with the sides of adjacent pieces so that such sides will be joined together from the bottom upwards thereby forming an integrated plate, and applying a reinforcement to the back of the integrated plate, the reinforcement consisting of two layers, one layer being made of glass tripods resting on the rear face of the plate, and the other layer being made up of glass cups resting on the tripods.

'7. A method according to claim 6, wherein heat is applied to the assembly of plate, tripods and cups to cause the tripods to be fused to each other and to the plate and the cups to be fused to each other and to the tripods.

8. A method according to claim 6, wherein each tripod has three splayed legs united at the top to provide a substantially hemispherical seating for the purpose described.

9. A method according to claim 6, wherein each tripod has three splayed legs united at the top to provide a substantially spherical seating, the legs being joined by webs the lower ends of 10 which terminate above the lower ends of the legs.

10. A method according to claim 6, wherein each tripod has three splayed legs united at the top to provide a substantially spherical seating, the lower extremity of each leg being formed as part of a sphere.

11. A method according to claim 6, wherein each tripod has three splayed legs united at the top to provide a substantially spherical seating, the outer part of the lower extremity of each leg being formed with two chamfers for the purpose described.

12. A method according to claim 6, wherein each cup is of substantially triangular shape in plan, each corner of the triangle being formed with two chamfers for the purpose described.

13. A method according to claim 6, wherein each cup is of substantially triangular shape in plan, each corner of the triangle being formed with two chamfers, and a recess conforming to part of a sphere is provided at the bottom of each pair of chamfers for the purpose described.

14. A method according to claim 6, including the step of applying spring clips to the cups to hold them together on the tripods and then applying heat to the assembly of plates, tripods and cups to cause the tripods to be fused to each other and to the plate and the cups to be fused to each other and to the tripods, the spring clips being thereafter removed.

ERNEST JOSEPH JOBLING-PURSER. SHAUN MATURIN COX.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 519,186 Hughes May 1, 1894 1,206,177 Twyman, et a1 Nov. 28, 1916 

